Greetings,

I'm working on a project using a sensor which outputs angle data.
The aim of the project is to measure the change in sensor angle every
sample time, and from the difference in angle run a servo the right
direction and distance to keep a motorized platform on a constant heading.

The program has to detect the change in position between the current 
and the previous sample and output that difference and polarity of angle 
for further arithmetic to another couple of PIC's.

The 32-bit sensor data is loaded into a shift register then extracted by 
a 18f252 which does the subtraction for difference.

The part of the data used is in the format:
3 modulo bits then 23 bits of  angle.
eg:   XXXXXXMM:MDDDDDDD:DDDDDDDD:DDDDDDDD

The modulo bits, MMM, increment when crossing 359.999 to 0 deg and
decrement when crossing from 0 to 359.999 deg.

As bit 7 of byte 2 is a modulo bit and not part of the angle
the data D......D ranges from

359.99..deg              =7F FF FF
180                      =40 00 00
45                       =10 00 00
00                       =00 00 00
-00                      =7F FF FF
-180                     =40 00 00
-359.99..deg             =7F FF FF

The 360-0 deg rollovers need to be kept track of   to give the angle
polarity.

To get it started I have been using the least significant  modulo  bit
(=byte 2,7) and msb of data to detect polarity:

;------------------------------------------------------------------------ 


; arrange bytes for subtraction
; need 0xff in byte3 and 1 in modulo positions if modulo=1
; or m=0 and 0x00 in byte3 if modulo=zero
;------------------------------------------------------------------------- 



    movlw            b'11000000'
    andwf            G2,0          ;and G2,7-6 to WREG
    movwf            modulo        ;move to modulo  to W
    tstfsz           WREG
    bra              neg_ang       ;modulo bits present indicate neg
angle
    bra              pos_ang        ;or pos angle

;-------------------------------------------------------------------------
;make into angle suitable for subtraction: 2's comp of 4 bytes
;change byte3 to 0 =or 0xff
;change byte2,7-6-5 to 00 or 11
;---------------------------------------------------------------------------

pos_ang     clrf    G3
             bra     diff          ;go to subtraction

neg_ang     movlw    0xff
             movwf    G3           ;sets the MSB, G3,   either all
1's or 0's for the following 2's complement subtraction

Now this seems to  work up to a point, between -359 and 359, and has
allowed testing of the whole project, but when the sensor is turned more
than 360deg the output at the rollover is incorrect at that moment, and
the servo following the angle change twitches,  so I know there's a
problem.

Here's a little table of the modulo  and the msb of data as the sensor
is revolved

DEG/MOD/DATA msb
2881  000  0
2879  111  1
2521  111  0
2519  110  1
.
.
1079  010  1
  721  010  0
  719  001  1
  361  001  0
  359  000  1
   +0  000  0
   -0  111  1
-359  111  0
-361  110  1
-719  110  0
-721  101  1
-1079 101  0
.
.
-2519 001  0
-2521 001  1
-2879 000  0
-2881 111  1

Appreciate any clues of how to get the change of angle from this data.

TIA

Brian Harris







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